Field of the Invention
The present invention relates to a radiation detection apparatus which detects radiation and a radiation imaging system which obtains a radiation image by using the radiation detection apparatus.
Description of the Related Art
Recently, a flat panel type radiation detection apparatus has been popularized, which uses a sensor array having conversion elements, each designed to convert radiation into an electrical signal, arranged in a two-dimensional array. Such a sensor array generally includes a conversion element formed on a glass substrate for each pixel and a switch element such as a TFT which transfers an electrical signal converted by the conversion element to the outside. These elements are two-dimensionally arranged in an array. Japanese Patent. No. 4018725 discloses a technique of obtaining an image by using such a sensor array. According to this technique, a plurality of gate drivers are arranged outside or on a glass substrate to drive switch elements via driving signal lines. In addition, like the gate drivers, a plurality of sense amplifiers for charge detection are arranged outside or on the glass substrate to detect electrical signals extracted via image signal lines. An image is then formed from these detected electrical signals.
Such a radiation detection apparatus has the following problems because conversion elements detect slight charges. For example, an imaging room in a hospital or the like is equipped with an apparatus for emitting radiation, another diagnostic apparatus, or the like together with a radiation detection apparatus. These apparatuses sometimes use high power. That is, there can be an environment where an apparatus which detects slight charges and an apparatus which uses high power coexist. In such an environment, unwanted electromagnetic energy from a high power apparatus becomes magnetic field noise to other apparatuses. This sometimes causes malfunction or performance deterioration in these apparatuses. When alternating current magnetic field noise is externally applied to the radiation detection apparatus, image noise in a horizontal stripe pattern called line artifacts appear in an obtained image. Such noise is generated from high power equipment, the inverter of an X-ray generator, and the like, in particular, and has a frequency band ranging from about 1 kHz to about 100 kHz. In addition, such alternating current magnetic field noise can arrive from various directions depending on the installed or used states of a radiation detection apparatus and high power equipment. It is generally difficult to take anti-noise measures against such alternating current magnetic field noise.
Conventionally, various techniques for reducing image noise caused by such an alternating current magnetic field have been proposed. Japanese Patent Laid-Open No. 2012-119770 has disclosed a technique of eliminating, from a final image, the influence of an externally arriving alternating current magnetic field with a specific frequency and a specific amplitude by subtraction processing by adjusting readout times for a dark image and a radiation image. In addition, Japanese Patent Laid-Open No. 2012-112726 has disclosed a technique of reducing the influence of electromagnetic noise by arranging a conductive member, a photoelectric conversion unit, and a scintillator in the order named from a side of a radiation detection apparatus which is irradiated with radiation.
The technique disclosed in Japanese Patent Laid-Open No. 2012-119770, however, cannot eliminate image noise by subtraction processing when the frequency or amplitude of an alternating current magnetic field changes at the time of obtaining a dark image and at the time of obtaining a radiation image. In addition, in order to make the time from the start of obtaining a dark image to the start of obtaining a radiation image become an integer multiple of an external magnetic field cycle, it is necessary to delay an image obtaining interval in accordance with the external magnetic field cycle, resulting in a decrease in imaging speed. Furthermore, although the technique disclosed in Japanese Patent Laid-Open No. 2012-112726 can reduce electromagnetic noise from a radiation incident direction, it is difficult to obtain an effect with respect to an alternating current magnetic field arriving from the horizontal direction at the radiation detection apparatus.
The present invention reduces image noise caused by an alternating current magnetic field arriving from the horizontal direction with an unknown frequency and amplitude in a radiation detection apparatus.